Stretch activation (SA) is an intrinsic sarcomeric property that increases muscle force, power and economy. SA is most prominent in muscle types that rhythmically lengthen and shorten such as cardiac and insect flight muscle. Rapidly increasing the length of a muscle with significant SA causes a delayed jump in force above calcium activated force. The mechanisms behind this force increase are not known, thus limiting our understanding of a fundamental muscle property. Our long-term goal is to apply insights gained from learning how SA mechanisms modulate force, power and muscle efficiency to help devise ways to restore impaired muscle function. The immediate objective of this application is to elucidate the sarcomeric mechanisms behind SA. Our central hypothesis is that SA can be caused by any sarcomeric mechanism that increases the total number of strongly bound cross-bridges following stretch. We propose that there are at least two mechanisms by which this occurs. In moderately SA muscle types the increase occurs by a myosin based mechanism, while a thin filament mechanism is required in highly SA muscle types. Our hypotheses are based on our novel preliminary data that a myosin isoform exchange increases SA force generation in a minimally SA muscle type to be equivalent to a moderately SA muscle type, and that a specific troponin C isoform, TnC4, is necessary for SA in highly SA insect flight muscle. These findings were made possible by our development of a new Drosophila muscle fiber preparation, the jump muscle, which allows us to look for gain of SA function and not just loss of SA function in the IFM. Specific aim 1 is to determine the kinetic and structural mechanisms by which some myosin isoforms enhance SA force production. Our working hypothesis is that for SA myosin isoforms, muscle stretch increases their probability of temporarily rejoining other cross-bridges in a low force state, thus increasing the number of cross-bridges available to subsequently transition into a high force, actin bound state. We will test our hypotheses that Pi affinity and different versions of the myosin relay helix are critical for changing the sensitivityof myosin to stretch. Aim 2 is to determine mechanisms by which thin filament proteins contribute to SA. We will test our working hypothesis that TnC isoforms in SA muscle types do not fully activate the thin filament upon calcium binding compared to TnC isoforms from muscles with minimal SA. This allows for further activation of the thin filament by stretch. We will test the hypothesis that further activation occurs by direct physical movement of troponin and tropomyosin by troponin bridges which span the thick and thin filaments. The proposed research is significant because it will provide a detailed understanding of the kinetic and structural mechanisms by which myosin, TnC, and other muscle protein isoforms enable SA. We will gain new insights into fundamental mechanisms by which force, power, and energetics are modulated in different muscle types. Elucidating SA mechanisms may lead to ways of restoring or improving muscle function.